Rick,
Is Windows XP 100 percent compatible with Windows 2000? And will
everything that runs on Windows 2000 run on Windows XP?

I don't mean to pick on your use of "100 percent compatible". Being compatible means the two things are not the same. Otherwise, they'd have been identical 8^).

Having said that, i'd share my personal experience of not having found user-mode applications that work on Windows 2000 and broken on Windows XP (on x86 platforms). If you have run into a different set of experiences, i'm sure Microsoft would want to hear about it.